Metallized Film Capacitor Lifetime Evaluation and Failure Mode Analysis

Metallized Film Capacitor Lifetime Evaluation and Failure Mode Analysis R. Gallay Garmanage, Farvagny-le-Petit, Switzerland Abstract One of the main concerns for power electronic engineers regarding capacitors is to predict their remaining lifetime in order to anticipate costly failures or system unavailability. This may be achieved using a Weibull statistical law combined with acceleration factors for the temperature, the voltage, and the humidity. This paper discusses the different capacitor failure modes and their effects and consequences. Keywords Metallized film capacitor; failure mode; lifetime. 1 Capacitor technologies The following different power capacitor technologies are used in inverters: – Electrolytic capacitors characterized by very big capacitance per volume unit, but with low rated voltages and very important power losses due to the ionic conductivity. In particular, the bigger the capacitance density, the lower the rated voltage. – Film foil capacitors made of dielectric films between two plain aluminium foils. These capacitors can sustain very high currents. – Metallized film capacitors, which are made with dielectric films with a metallic coating on the surface. With this technology the electric-field stress may be much bigger than with film capacitors thanks to the metallization self-healing capability. Today the dielectric films that are used are mainly polypropylene (PP) or polyethylene terephthalate (PET). Formerly, paper (PA) was used in film foil technology—either pure paper or mixed with polypropylene (DM). In special applications, where high temperatures are required, polyethylene naphthalene (PEN) up to 125°C or polyphenylene sulfide (PPS) up to 150°C are used. PET presents the following advantages over PP: a dielectric constant 50% bigger (ε = 3.3 versus 2.2), which means 50% more capacitance in the same volume, a better mechanical resistance (which means a higher endurance to self-healing), and the possibility of manipulating thinner films, consequently leading to a smaller capacitance and a higher exploitation temperature (+10°C). The negative point is that the loss factor is ten times larger, which means a ten-fold increase in temperature elevation for the same rated power. The nominal electrical field is about the same. The capacitive elements must be dried to remove moisture, which would cause accelerated aging and bigger losses if left in the capacitor. In the case of power capacitors, the dried elements are either impregnated with vegetable oil or with gas (SF6, N2, etc.). The dielectric films are either wound or stacked before being inserted in a plastic or metallic container. The best winding machines are required to produce active wound elements of reliable quality in the case of oil-free capacitors. One way of overcoming the difficulty of controlling the space ratio between the gas and the film in the winding curves is to wind the film on a large-diameter wheel and to cut the film layers to obtain a stack. The plastic containers are not completely moisture tight—there is always some residual permeability in polymers. In the case of metallized films, this may lead to electrode corrosion when the capacitors are submitted to environmental conditions of high humidity. The electric-field stess in metallized film capacitors may be much larger than in film foil capacitors. This is obtained thanks to the ability of the electrodes to self-heal. If a breakdown occurs in the polymer, the current will increase through the defect and on the electrode near the defect. Close to the defect the current density will be big enough to evaporate the 100 nm metallic layer. If the capacitor is well designed, the phenomenon will stop when the diameter is large enough to insulate the defect and small enough not to damage the film. The electrode resistance (given in ohm/square) is the key parameter to define to achieve good self-healing behaviour, with Joule losses as small as possible. A thick metallized layer will present a lower resistance, but higher energies will be involved during the self-healing process, leading to greater damage [1-5]. 2 Capacitor failure modes Most of the metallized film capacitors fail because the capacitance drops below the required tolerance. This normally occurs after the expected lifetime given by the manufacturer. The capacitance drop is generally accompanied by an increase of the loss factor. From a general point of view, the causes of capacitor failures may occur because of bad design, bad processes, or inappropriate application conditions. During the design phase, the following causes may lead to failure: the dielectric film is too thin, insulation distances are too small, the metallization layer is too thick or too thin, or the conductor is the wrong size. During production, causes may include the following: poor mechanical tension control during the winding, bad drying (leaving too high a humidity content in the capacitor), or bad sealing. In application, the causes may be: higher voltages, EMI, lightning, higher temperature, or a high humidity environment. The failure modes are a little more complicated to describe because different causes may lead to the same modes. Figure 1 gives a non-exhaustive summary of the possible failure modes which can occur in metallized film capacitors. Fig. 1: Metallized film capacitor failure modes with their causes, effects, and consequences For example, bad space factor control of the dielectric films during the winding operation will be the cause of the electrode corona demetallization, which will lead to a fast capacitance drop and to the loss of functionality of the capacitor. A bad choice of the metallization resistance value, or poor metallization control during the film manufacturing process, leads to bad self-healing management, which may damage the dielectric film mechanically and produces heat which is transmitted locally to the next film layers. At this location the dielectric strength of the film drops and breakdown may occur. Consequently, chimneys of melted polypropylene may appear through the winding. The formed channel is conductive, inducing a drop in the insulation resistance and a leakage current that can generate enough heat to melt the polypropylene and increase the internal pressure of the capacitor. Along with bad metallization resistance, the final consequence can, in the worst case, lead to fire ignition or even a capacitor explosion. Fig. 2: Chimney through the film layers in the capacitor winding One of the main failure modes is often due to high currents, which increase the capacitor temperature, leading to a reduction of the breakdown voltage and, in the worse cases, even melting of the capacitor. In this regard, the shape of the capacitor is very important. For high-power applications, it is important to build short elements in order to reduce the current path length and increase the number of parallel layers, and consequently reduce the heating. The current capability of a capacitor is specified through the series resistance Rs and the loss factor tan δs at different frequencies. The relation between the two factors, in the high-frequency domain where the effect of the insulation resistance is negligible, is given by the linear relation ZR tanδωss= = RC, (1) Zi where C is the capacitance, ω = 2π f is the frequency, ZR is the impedance real part and Zi the impedance imaginary part. The presence of humidity in the capacitor, because of poor drying during the manufacturing process, or because the moisture permeability of the material was too high, or because the humidity level where the capacitors are installed was too high, may lead to three failure modes with different effects and consequences. Fig. 3: Electrode corrosion due to the presence of moisture The first is electrode corrosion (see Fig. 3) [6-8], where the series resistance will slowly increase over time. The effect is a loss factor increase due to the electrode thickness reduction and a heat dissipation increase. The elevation of the temperature will accelerate the capacitance loss because of the reduction of the dielectric strength with temperature, ending with the loss of functionality of the capacitor. The second effect (see Fig. 4), today known as ‘corona’ [9-11], is due either to a decrease of the dielectric strength of the gas present in the capacitor in the gaps between the dielectric films or to a poor space factor control of the films. The bigger the gap, the more severe the problem. The thickness of the gap is characterized by the space factor, which is the ratio of the dielectric thickness to the total distance between the electrodes. This space factor is very difficult to control in curves of flat windings, leading manufacturers to build either round winding or stacks. Only performant winding machines can achieve good space factor control by managing the mechanical tension of the films during the winding. The consequence of this is a fast capacitance decrease due to the appearance of corona discharges on the electrode edges, i.e., the locations where the electrical field is more intense due to the point effect. In the case of segmented metallization, the corona failure mode may also propagate from the non-metallic lines which separate the active electrode metallic areas. Fig. 4: Demetallized electrodes by corona arcing in the gas gap between the films The third failure mode is a reduction of the insulation resistance, which is the parallel resistance of the capacitor. A decrease in insulation resistance leads to an increase in current leakage from one electrode to the other. This phenomenon is present at low frequency. It may be measured via either the loss factor (tan δ) or the d.c. resistance Rp. The relation between the two parameters is given by the following relation (only true at very low frequencies): ZR 1 tanδp = = . (2) Zip RCω This later failure mode may have a runaway behaviour. The more the insulation resistance decreases, the more heat is produced, and the more the temperature increases, which leads to a new insulation decrease.

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